1. Field of the Invention
These inventions relate to mass spectrometers, for example quadrupole mass filter spectrometers.
2. Related Art
Mass spectrometers are used in atomic and chemical analysis to determine the quantity and atomic or chemical makeup of unquantified or unknown atoms and compounds. There are a number of different types of mass spectrometers, but the following discussion will focus on quadrupole mass spectrometers as a particular application of the inventions to spectrometers. One or more of the inventions could be applicable to other mass spectrometers, including multi-pole spectrometers.
A quadrupole mass spectrometer system generally consists of a source of ions, a quadrupole mass filter, an ion detector and associated electronics. A gaseous, liquid or solid sample is ionized in the ion source and a portion of the ions created in the ion source is injected into the quadrupole mass filter. The filter rejects all ions except those in a selected mass-to-charge ratio (mass/charge) range as determined by the system electronics. (It will be understood from the context herein where the references to mass without mentioning charge refer to the mass-to-charge ratio, as appropriate, even though charge is not specifically expressed, because the effect of the field depends on the charge of the ions.) That selected mass range is usually less than 1 atomic mass unit (AMU) centered at a particular mass. Because the masses of the elements making up the sample are often unknown, the system varies the mass range from a starting mass number to an ending mass number to test for and sense particles having the masses within the mass range selected. The mass range can be as low as one AMU up to thousands of AMU. The system operates either automatically or under manual control. The mass analysis of the composition of the sample is performed by rapidly scanning the DC and RF voltages, or the frequency of the RF voltage, on the quadrupole filter, thereby scanning through the possible masses and recording the abundance of each as transmitted through the filter.
The effectiveness of a mass spectrometer system is determined in large part by its sensitivity and selectivity, the latter usually being called resolution. Sensitivity determines how small a quantity of sample can be detected and its constituents quantified. Resolution must be sufficient for two adjacent mass peaks to be clearly separated such that their separate characteristics can be determined.
A conventional quadrupole mass filter consists of four conductive rods arranged with their long axes parallel to a central axis and equidistant from it. The cross sections of the rods are preferably hyperbolic, although rods of circular cross section ("round rods") are common. Round rods will be referred to and shown for simplicity, but it should be understood that other conventional rods are equally applicable. To select which ions are rejected and which are passed through the mass filter, a selectable voltage .+-.(U+V cos .omega.t) is applied on adjacent rods, so that opposite rods have the same potential and adjacent rods have equal but opposite potentials. U is the DC or offset voltage and V is the radio frequency (RF) component of the voltage applied to the quadrupole rods, at a given frequency .omega. and time t. The field created within the region surrounded by the rods is a quadrupole field, with the electric field sensed by the ions travelling between the rods directly proportional to the distance from the central axis.
Ions injected into the entrance of the filter will exhibit oscillatory trajectories generally in the direction of the central axis (Z-axis). Those ions that oscillate too far from the central axis (in the X-axis and/or in the Y-axis directions) will, in general, not pass through the filter, while those ions that exhibit relatively short oscillatory trajectories pass from the exit of the filter and are detected. The extent of the oscillatory trajectories for a given ion mass is determined by the selected voltage. The selected voltage comes from a certain set of pre-determined voltages that are a function of the mass of the ions. The pre-determined voltages are typically developed empirically for the particular mass spectrometer configuration, and are stored in a computer or other processor memory as a look up table or equation for use during operation of the system. The magnitudes and ratio of the DC and RF components of the applied voltage can be adjusted such that only a very narrow mass range of ions will pass through the device. The narrower the mass range of the ions passing through the device, the higher the resolution, and the easier it is to distinguish ions of similar masses. Sweeping the RF voltage with a fixed RF/DC ratio will result in a mass spectrum over the range of masses selected for analysis.
The resolution of the mass filter can be increased by decreasing the RF/DC voltage ratio, at least until a ratio is reached such that ions are no longer transmitted through the filter. However, as resolution is increased the ion transmission decreases. The transmission is the fraction of input ions of the same mass that make it through the filter. With lower transmission, the amount of the sample becomes more important and it may be more difficult to quantify the results for each mass peak in a spectrum. The resolution achievable depends on the mass of the ion and the length of the mass filter, and the transmission depends on the resolution and the input conditions of the ions, i.e., on the positions and velocities of the ions as they enter the filter. Other factors affect the operation of the mass filter, such as fringe fields at the ends of the mass filter, the presence or absence of focusing elements, and the voltages that may be applied to these focusing elements. While many of these factors are understood, there is still room for improvement in the resolution and sensitivity of mass filter spectrometers.
One area of improvement is in the transmission of the ions for a given resolution, or conversely increasing the resolution while still ensuring a desired level of transmission. Because of the number of ions that are lost before and after entering the mass filter, analysis often uses more time and/or larger samples to achieve the desired results. It is well understood that ions traveling along the central axis (Z-axis) or not very far off the axis are easily transmitted through the quadrupole mass filter. However, the loss often occurs near the entrance to the quadrupole mass filter due to ions not having the required properties of position and velocity to match the electric field of the quadrupole mass filter existing at the time the ion approaches it. One reason may be that the ion starts too far away from the central axis to be brought back before it collides with the rods of the quadrupole mass filter. Another reason may be that the ion's velocity moving away from the central axis is too great to be brought back to the axis of the quadrupole mass filter by the effects of the electric field. Moreover, the electric field that might bring an ion into the mass filter varies over time, as can be seen from the expression .+-.(U+V cos .omega.t) . The variation is sinusoidal with a frequency .omega., which can be in the megaHertz range, so at one time an ion with a given position and velocity may make it into the mass filter but not at another time less than a millionth of a second later or earlier. Only half a cycle or a full cycle later will an ion of the same position and velocity be able to pass through the mass filter. At a given time, the ion positions and velocities that will gain them entrance to the mass filter are depicted in the FIGS. 3 and 4, and ions having positions and velocities outside the particular ellipse corresponding to the applicable time are lost. Therefore, ion transmission and/or mass filter resolution are lower than desired.